Cyclic AMP (cAMP) in mammalian cells functions by binding to cAMP receptor protein, the regulatory subunit of cAMP-dependent protein kinase. The cAMP receptor protein has two different cAMP binding sites, and cAMP analogs that selectively bind to either one of the two binding sites are known as site A-selective (C-6 analogs) and site B-selective (C-2 and C-8 analogs). We discovered that site-selective cAMP analogs exhibit potent growth inhibition in vitro and in vivo in a range of human carcinomas, fibrosarcomas, and leukemias without causing cytotoxicity. Site-selective cAMP analogs are also promising in view of their ability to act synergistically as non-toxic differentiation agents at low, micromolar doses, not only with each other, but also in combination with other differentiating agents currently accepted for clinical use. 8-Cl-cAMP, the most potent site-selective cAMP analog, acts synergistically with granulocyte-macrophage colony-stimulating factor in the differentiation of HL-60 human promyelocytic leukemia cells. 8-Cl-cAMP also holds clinical promise for immunodiagnosis and immunotherapy. 8-Cl-cAMP acts alone and synergistically with interferon gamma (IFN-gamma) to selectively increase expression of carcinoembryonic antigen in IFN-gamma- insensitive LS-174T human colorectal carcinoma cells in vitro and in vivo. 8-CI-cAMP also exerts potent growth inhibition of both p-glycoprotein (pgp) -associated and -unassociated multidrug resistant human cancer cell lines. The molecular mechanism for such potency in the growth inhibitory effect of 8-Cl-cAMP and other site-selective cAMP analogs takes advantage of the ability of these analogs to selectively modulate two isoforms of cAMP receptor proteins, type I and type II protein kinase, the positive and negative regulators of cell growth and differentiation. 8-Cl-cAMP markedly down- regulate the growth stimulatory protein, type I protein kinase while upregulating the growth inhibitory protein type II protein kinase. Site-selective cAMP analogs thus provide new biological tools for investigating cell proliferation and differentiation and also for the improved management of human cancer. 8-Cl-cAMP is now in preclinical phase I studies at NCI.